The present invention relates to a variator disc mounted in a toroidal continuously variable transmission and a method of fabricating the same.
FIG. 6 is a schematic diagram showing a variator for use in a toroidal continuously variable transmission mounted in vehicles and the like. The variator 10 includes an input shaft 13 driven into rotation by a vehicular power source 12. The input shaft 13 carries an input disc 15 on respective places near opposite ends thereof. These input discs 15 are each formed with a spline hole 15a at a center thereof, the spline hole formed with plural lines of spline grooves. The input disc 15 has the spline hole 15a meshed with a spline shaft 13a of the input shaft 13 so that the input disc is rotated in unison with the input shaft 13 as permitted of a minor movement axially of the input shaft 13. The input disc 15 is further formed with a concave race 15b on one side surface thereof.
An output portion 18 of the variator 10 is relatively rotatably carried on an axially central portion of the input shaft 13. The output portion 18 includes an output member 19 and a pair of output discs 20 integrally rotatably carried on the output member 19. The output disc 20 is formed with a concave race 20b on one side surface thereof in face-to-face relation with the race 15b of the input disc 15. The output member 19 is formed with a spline shaft 19c on each of the outer peripheries of opposite ends thereof. The spline shaft 19c is meshed with a spline hole 20c formed at the output disc 20. The output member 19 is further formed with a sprocket gear 19a on an outer periphery of its central portion, the sprocket gear meshed with a power transmission chain 23. Three disc-like rollers 21 are interposed between the race 15b of the input disc 15 and the race 20b of the output disc 20 in face-to-face relation. The rollers 21 are in rolling contact with the races 15b, 20b, as arranged with equal circumferential spacing. Each of the rollers 21 is rotatably carried by a carriage 22, and is adapted to be adjusted for its position relative to the races 15b, 20b by means of the carriage 22.
Thus, the variator 10 is configured as a so-called double cavity type which includes a pair of sets, each of which consists of the input disc 15, output disc 20 and rollers 21. The variator is arranged such that the input discs 15 transmit torque to the output discs 20 via the six rollers 21. The variator is adapted to vary the speed of rotation (transmission gear ratio) of the output discs 20 by adjusting the positions of the six rollers 21 by means of the carriages 22 (see two-dot chain line in FIG. 6).
The input disc 15 is formed from, for example, a bearing steel. The input disc is generally fabricated by the steps of heat treating a blank for hardening, the blank formed with the spline hole 15a and the concave race 15b by machining; finishing the spline hole 15a by broaching; and then finishing the race 15b by machining or grinding using an inside periphery (surface of minimum inside diameter) of the spline hole 15a as a working reference.
The variator 10 is required to maintain the individual rollers 21 in contact with the races 15b, 20b of the discs 15, 20 at high contact pressure in order to ensure the torque transmission between the input discs 15 and the output discs 20. Hence, it is a general practice to specify Vickers hardness (Hv) of the race surfaces 15b, 20b to be 700 or more thereby ensuring the fatigue strength of the races 15b, 20b. 
However, the heat treatment of the discs 15, 20 generally includes the steps of charging the blank in an oven for heating the whole body of the blank in high temperature atmosphere; quenching the blank; and tempering the same. Accordingly, the overall surface of the blank is hardened to the hardness of not less than Hv700 which is required of the races 15b, 20b. In a case where the spline hole 15a, 20c of the disc 15, 20 is subjected to an excessive load resulting from unbalanced load or the like, the spline hole 15b, 20b may develop cracks at its bottom and the cracks may further develop to cause fracture of the disc 15, 20 at worst. Furthermore, since the spline holes 15b, 20b are low in machinability, it is difficult to broach the heat treated spline holes 15b, 20b. This results in the increase of the machining cost.
It is an object of the invention to provide a variator disc capable of preventing the occurrence of cracks in its spline hole.
It is another object of the invention to provide a variator disc facilitating the finishing of the heat treated spline hole.
It is still another object of the invention to provide a fabrication method for variator disc which facilitates the fabrication of the inventive disc.
A variator disc in accordance with the invention is used in a variator of a toroidal continuously variable transmission, the disc including a concave race at least on one side surface thereof for rolling movement of rollers; and a spline hole at its center which is meshed with an input shaft; and having a surface hardness of not less than Hv700 at the race; and a surface hardness of not more than Hv600 at the spline hole; wherein at least a thin portion between the spline hole and an inside circumferential edge of the race is varied in hardness between a hardened layer of a bottom of the spline hole and a hardened layer of the race by an amount of not less than Hv20 per 1-mm depth.
According to the variator disc thus arranged, even if the spline hole is subjected to an excessive load, the occurrence of cracks at the bottom of the hole can be prevented because the spline hole has the surface hardness of not more than Hv600. Furthermore, the spline hole is easy to finish by machining and hence, the machining cost therefor is reduced. In addition, at least the thin portion is varied in hardness between the hardened layer of the bottom of the spline hole and the hardened layer of the race by the amount of not less than Hv20 per 1-mm depth. Because of such a great rate of the hardness variation, the thin portion need not be increased in thickness but the hardened layer of the spline hole can be decreased in hardness while a sufficient hardness of the hardened layer of the race is maintained as it is. Accordingly, the race has a sufficient depth of hardened layer in the thin portion, as well, so that the race can exhibit a favorable durability. If the amount of hardness variation is less than Hv20 per 1-mm depth, the thin portion must be increased in thickness in order to achieve an adequate hardness of the hardened layer of the race. This leads to the need for accordingly increasing the outside diameter of the disc. According to the invention, however, the hardened layer of the spline hole can be decreased in hardness without entailing an unwanted increase of the outside diameter of the disc.
A method of fabricating the variator disc of the invention comprises the steps of: heat treating a blank for the disc for adjusting a surface hardness thereof to not less than Hv700; annealing the spline hole by high frequency heating while cooling the race; and finishing the spline hole and the race.
According to this method, the spline hole of the disc is annealed while cooling the race of the disc. Therefore, the race is prevented from being decreased in hardness by the thermal influence of the annealing of the spline hole. In addition, at least the thin portion can be effectively varied in hardness between the hardened layer of the bottom of the spline hole and the hardened layer of the race. Accordingly, the disc of the invention can be fabricated easily.
According to the method of fabricating the variator disc, it is preferred that a cooling jig is positioned in parallel with the race as defining a minor gap therebetween and that the race is cooled by circulating a coolant through the cooling jig. This provides for an effective decrease of the thermal influence on the race during the high frequency heating of the spline hole.
It is preferred that the minor gap between the race and the cooling jig is in the range of 0.3 to 0.5 mm. In this case, the race can be cooled in an easy and stable manner.